Modern gas turbine annular combustors are usually provided with an upstream endwall or bulkhead which extends radially between inner and outer wall members to define an upstream plenum and a downstream combustion chamber. The bulkhead is usually provided with a plurality of circumferentially spaced apertures, each of which receives an air/fuel injection device for introducing a mixture of air and fuel into the combustion chamber during engine operation.
In order to protect the bulkhead from the direct effects of the combustion process it is often necessary to attach heatshields to the bulkhead structure. In a known arrangement the bulkhead is protected by an annular array of segmented heatshield elements. The segments, which are each associated with one of the air/fuel injection devices, extend both radially towards the inner and outer extents of the bulkhead and circumferentially to abut adjacent segments. The air/fuel injection devices extend into the combustion chamber through corresponding apertures in the heatshield segments. Each heatshield is spaced apart from the bulkhead so that a narrow cooling passage is defined between the two components. In use cooling air is directed into these passages to cool the bulkhead and heatshield components, and then exhausted to the combustion chamber.
In practice it has proved difficult to cool the region of the heatshield immediately surrounding the fuel nozzle aperture without using excessive amounts of cooling air. For structural reasons the various cooling arrangements present in the segments generally become discontinuous in this region. As a result the region surrounding the aperture tends to experience lower levels of cooling and as a result higher temperatures during engine operation. In the prior art arrangements this has lead to excessive thermal gradients being developed in the segments during high temperature combustor operation and, by virtue of the resultant radial temperature distribution, excessive circumferential hoop compression stresses in the region surrounding the fuel nozzle aperture. This has lead to premature failure of the segments in the region of the fuel nozzle aperture by thermal fatigue.
The present invention, therefore, has for one of its objectives to improve the fatigue life of combustor heatshield components, and as a second objective to reduce the amount of cooling air required to cool these components.